The present invention generally relates to a piezoelectric resonator and more particularly, to an improved strip-shaped or rectangular piezoelectric ceramic resonator which utilizes a length vibration mode of a piezoelectric ceramic plate in a strip or rectangular configuration.
Conventionally, a strip-shaped or rectangular piezoelectric ceramic resonator(referred to as a rectangular piezoelectric ceramic resonator hereinafter), has been formed as a resonator R as shown in FIGS. 1(a), 1(b) and 1(c), which includes a rectangular piezoelectric ceramic plate 1 having an approximately constant width. In its one main surface is formed a concave groove 2 axially extending from one end to the other end thereof along its central portion in the widthwise direction so as to provide split electrodes 4 and 5 on said main surface 1a divided by said groove 2. An entire face electrode or common electrode 6 is formed over the other main surface 1b of the piezoelectric ceramic plate 1, with said piezoelectric ceramic plate 1 being subjected to polarization from the side of the entire face electrode 6 towards the split electrodes 4 and 5 in a direction as shown by symbols P.
Incidentally, in the known piezoelectric ceramic resonator R as described above, disadvantages arise because an attenuation pole is provided at a low frequency side. Therefore selectivity at a high frequency side is inferior as compared with that at the low frequency side. Also, when the piezoelectric ceramic resonator is combined with an intermediate frequency transformer or more than two piezoelectric ceramic resonators of this kind are combined with each other, such imbalance in the sensitivity is increased, while comparatively strong spurious responses of a longitudinal vibration resonating in the widthwise direction of the piezoelectric ceramic plate 1 are produced. Thus, in the case where the piezoelectric ceramic resonator is applied, for example, to a short wave broadcasting receiver, beat troubles tend to take place due to such spurious responses.